|BERKELEY, CA Using the
One-Ångstrom Microscope (OÅM) at the National Center for Electron Microscopy (NCEM),
researchers at the Department of Energy's Lawrence Berkeley National Laboratory have made
unprecedented images of columns of carbon atoms in a diamond lattice, only 0.89 angstrom
apart less than one ten-billionth of a meter.
For the first time, moreover, an electron microscope has been able to resolve nitrogen atoms in the presence of more massive gallium atoms in gallium nitride, in columns spaced only 1.13 angstroms apart.
"The ability to make images of light elements such as carbon, nitrogen, and oxygen in solids at atomic resolution is a very big step forward and it was achieved by a technique that can be a routine tool in the future. Therefore, it is of great interest to science and industry," says Christian Kisielowski, who with Michael O'Keefe and their colleagues Christian Nelson, Chengyu Song and Roar Kilaas of Berkeley Lab's Materials Sciences Division recently announced the record-breaking result.
Many of the most promising solids under investigation today, including superhard materials, high-temperature superconductors, and semiconductors with large band-gap energies, incorporate light elements in crystal lattices at short interatomic distances.
"Seeing small atoms at atomic resolution has always been a challenge, because they don't strongly scatter the electrons in the microscope's beam," says Michael O'Keefe. "When the light atoms are close to heavy ones, it has been virtually impossible to resolve them. Heavy atoms scatter electrons much more, and as a result the interference pattern is just too complex to resolve."
Kisielowski explains that "the OÅM overcomes this difficulty by making a through-focal series of images in the case of the gallium nitride, 20 different images, each with the scattered electrons interfering with different relative phases and then uses computer processing to unscramble the electron waves and combine them into a single high-resolution image in which all electrons are in phase." He adds, "It's a way of going from the complexity of the lattice images produced by the OÅM to the simplicity of crystalline structures."
The OÅM had its genesis in the early 1990s, when NCEM's three-story, million-volt Atomic Resolution Microscope, or ARM, was the world's most powerful, with a practical resolution of 1.6 angstroms though Kisielowski once managed to squeeze out 1.54 angstroms. Then a Japanese-built, one-and-a-quarter-million-volt machine in Germany achieved 0.95-angstrom resolution, but at a cost of more than 10 million Deutschmarks.
At about the same time, O'Keefe proposed a way to computer-process through-focus images to achieve higher resolution from a medium-voltage microscope, an approach first suggested in the late 1960s. "Such a microscope can be designed so that its information limit' the limit to which it produces phase-scrambled information lies well beyond its traditionally defined nominal resolution, with all the transferred information in phase," he explains. "By combining information from many images, a single image with resolution approaching the information limit can be achieved in practice."
Electron beams are the basis of all transmission electron microscopy, and through-focus methods depend upon beams with all electrons at nearly the same energy beams with very little "energy spread." Not until the early 1990s did field-emission beam sources become stable enough for medium-voltage instruments to operate reliably.
Thus when a group of researchers working in the European Commission's BRITE-EURAM program set out to build a new generation of high-resolution electron microscopes using medium voltages, they invited NCEM to be a partner in the project, based on NCEM's high-resolution expertise and O'Keefe's theoretical contributions. In 1993, NCEM was able to secure the funds to acquire a suitable instrument, a Philips CM300.
Although a typical CM300's resolution limit is 1.7 angstroms, O'Keefe laid out specifications that would optimize the instrument's information limit. Recent results confirm the OÅM's capacity to produce phase-scrambled information far beyond 1.7 angstroms. In the case of diamond, Kisielowski and O'Keefe, working with Y.C. Wang, have shown that the OÅM's information limit can extend to at least 0.89 angstrom.
And as planned, powerful computer programs used to process the focal-series images have allowed OÅM to reconstruct images with resolutions near its information limit.
Meanwhile the ARM, NCEM's "grandfather" microscope, is far from being outmoded by its diminutive descendant. The OÅM can only produce ultra-high resolution with samples less than a hundred angstroms thick, which are prepared by planing away layer after layer of atoms, using a low-angle, low-energy beam of argon atoms in an "ion mill" until the samples are "close to being all surface," O'Keefe jokes.
Kisielowski stresses that "sample preparation is getting to be a bottleneck. It's a nasty job, and nobody wants to do it, because you don't get to be a professor that way."
The ARM can use samples that are three times thicker and composed of heavy atoms, yet still achieve a respectable resolution. A high-voltage microscope can accommodate larger sample holders, which are required to perform dynamic experiments such as in-situ straining or heating. It also allows for larger tilt angles than the OÅM, and, says Kisielowski, "material scientists love to observe matter from different angles different projections are the essence of any tomographic experiment, for example."
The ARM will see wide use for years to come. Today, however, the ultra high-resolution performance of OÅM is unsurpassed. The 1.13-angstrom resolution achieved with gallium nitride, allowing images of its nitrogen atoms as well as its gallium neighbors, stands as an extraordinary achievement but also as a challenge to Kisielowski, O'Keefe, and their colleagues.
Says Kisielowski, "We're aiming to investigate materials with even shorter bond lengths with the present information limit. We want to have procedures in place that work reliably and fast to make the experiments available to our user community as soon as possible colleagues from other laboratories have already started to share our excitement by investigating their own samples with the OÅM."
Uli Dahmen, head of NCEM, shares Kisielowski's enthusiasm. "This achievement is based on more than six years of team effort in planning, installation and testing. After all this time, it's a thrill to actually see it work. NCEM has reached a very important milestone." He adds, "The one angstrom barrier has been a Holy Grail for electron microscopists worldwide ... The OÅM makes a truly extraordinary addition to Berkeley Lab's scientific toolbox,' and I can't wait to see what new discoveries it will bring for our users."
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.